I struggle to name a gun in this game that's completely useless.
That's what makes this game so good.
This - most of the guns listed here can actually be good on the right build.
FatMan is uber - one shot everything
Silenced Pistol and Silenced 22 SMG - both are quite strong with a max crit build, better crits and the Professional perk. Don't knock if you have not tried it
Service Rifle - Fully upgraded it is a beast in the low levels. I have killed a group of three Death Claws with one before. Not sneak sniping
That Gun - hits hard, loads fast, high crits.
Personally, the guns I like the least is the 10MM SMG and the 357 Pistol. The 10mm SMG gets beaten by the 9mm and the 12.7mm SMG. The 357 Pistol reloads to darn slow. Even with Rapid Reload and a high agility. I will take a 10mm pistol over it any day.